1. Field of the Invention
The present invention relates to conductive paste used for forming a thick film electrode and the like, and a ceramic electronic part comprising an electrode formed by using the conductive paste.
2. Description of the Related Art
A typical example of an electronic part having a structure in which an external electrode is provided on a ceramic element assembly is a monolithic ceramic capacitor as shown in FIG. 1.
The monolithic ceramic capacitor comprises a ceramic element assembly (capacitor body) 13 formed by alternately laminating a ceramic layer 11 as a dielectric layer and an internal electrode 12, and then sintering the resultant laminate; and external electrodes 14 arranged on both sides of the ceramic element assembly 13 so as to cause conduction with the internal electrodes 12.
The internal electrodes 12 are formed by using a conductive paste which can be burned with ceramic and which contains a metal powder of Pd, Ag-Pd or the like as a conductive component.
The external electrodes 14 are formed by coating a conductive paste on the burned ceramic element assembly 13 and then burning the paste. As the conductive paste for forming the external electrodes (referred to as "the external electrode bodies" or "the thick film electrodes" hereinafter) 14 used has been a metal powder of Ag, Ag-Pd or the like, and zinc borosilicate glass frit dispersed in an organic vehicle.
In order to improve the heat resistance and solderability of the external electrodes, the surfaces of the external electrode bodies (the thick film electrodes) formed by baking the conductive paste are plated with nickel, tin or solder to form plating films of nickel, tin or solder on the thick film electrodes.
In order to prevent the occurrence of a tombstone phenomenon during mounting, a ceramic element assembly having small dimensions is often plated with solder to form solder plating films on the outermost layers of the external electrode bodies (the thick film electrodes).
The above-described monolithic ceramic capacitor is generally surface-mounted by soldering the external electrodes to a land of a wiring board. However, the solder plating film on the outermost layer is re-melted by the heat generated by soldering during mounting, and at the same time, the melted solder plating film is scattered due to bumping (violent escape) of the moisture trapped in the external electrode bodies during the plating step in the formation of the external electrodes, thereby causing a short-circuit with adjacent mounted electronic parts in some circumstances. The above-described problem is true for not only the case in which the outermost layer is plated with solder, but also the case of a ceramic electronic part in which a plating film of any of various low melting point materials such as tin and the like is formed.